1. Field of the Invention
The present invention relates to a display apparatus in which devices are driven based on data retained in auxiliary capacitors, and more particularly, to a display apparatus having a structure capable of obtaining a wide design margin in view of this point.
2. Description of the Related Art
In recent years, along with developments of information devices, needs for thin low-power consumption display apparatuses are increasing. Therefore, display apparatuses that meet those needs have been under active research and development. In particular, it is desirable to save power and space regarding a wearable personal computer (wearable PC) or an electronic notebook.
Many types of liquid crystals do not have a so-called memory property, so it is necessary to continuously apply a voltage to a liquid crystal during a display period. When, for example, the wearable PC is to be used in various environments, it is difficult to ensure reliability of a liquid crystal having the memory property.
An electrophoretic display apparatus has been proposed in U.S Pat. No. 3,612,758 as one type of a thin and light display system having the memory property. The electrophoretic display apparatus includes a pair of substrates arranged at a predetermined interval, an insulating fluid filled between the substrates, a large number of colored charged migration particles dispersed into the insulating fluid, and display electrodes provided in each pixel along the respective substrates.
In the electrophoretic display apparatus, colored charged migration particles are charged to one of a positive polarity and a negative polarity, so the particles are absorbed by any one of the display electrodes based on the polarity of a voltage applied between the display electrodes. A state where the colored particles are absorbed by an upper electrode so that the colored particles can be observed, and a state where the colored particles are absorbed by a lower electrode so that a color of the insulating liquid can be observed are switched based on the applied voltage to display various images. Such type of electrophoretic display apparatus is called a “vertical movement type”.
An In-Plain type electrophoretic display apparatus as illustrated in FIGS. 4A and 4B has been proposed in Japanese Patent Application Laid-Open No. H09-211499.
The electrophoretic display apparatus does not have a structure in which the insulating liquid is sandwiched between the substrates as in the case of the vertical movement type but has a structure in which each of first electrodes 31 is provided along an interpixel shielding layer and a second electrode (reflecting electrode) 32 is arranged on an entire pixel display portion and covered with an insulating film in order to reflect incident light.
The insulating liquid is transparent. As illustrated in FIG. 4A, the second electrode 32 is covered with migration particles 30 to perform black display. As illustrated in FIG. 4B, the migration particles 30 are collected near the first electrodes 31, each of which is located between adjacent pixels, to expose the second electrode 32, thereby performing white display. An image can be displayed by controlling the polarity of the applied voltage for each pixel.
In many display apparatuses such as a liquid crystal display apparatus, an electrophoretic display apparatus, and an organic electroluminescence (EL) display apparatus, a liquid crystal, an electrophoretic liquid, or an organic EL material is provided above a substrate in which TFTs are formed (hereinafter referred to as TFT backplane). At least one of electrodes for driving the liquid crystal, the electrophoretic liquid, or the organic EL material is provided on the TFT backplane. An electrode of each of the TFTs (drain electrode in many cases) is connected with an electrode of a corresponding display device.
When a display operation is to be performed, first, a TFT is turned on to apply a voltage to the electrode of the display device through the drain electrode. In order to retain the applied voltage even when the TFT is turned off thereafter, one of the drain electrode and the electrode of the display device is provided such that a capacitor is formed between another electrode opposing that electrode.
However, when an electrophoretic device in which particles are charged as described above is used or when a device having spontaneous polarization, which is represented by a ferroelectric liquid crystal is used, the following problem occurs.
That is, when the device is driven by a switching device such as a TFT in a state where charges are retained by a capacitor provided in each pixel, an amount of charges retained thereby is changed by a movement of the charges or by polarization inversion, to significantly reduce apparent voltage retention. As a result, it becomes difficult to maintain a desirable write state.
When data is to be written into each of pixels arranged in matrix, charges are stored in a capacitor (hereinafter referred to as auxiliary capacitor) provided in each of the pixels during a write time.
After charging, a pixel driving voltage is retained by the charges stored in the auxiliary capacitor of each of the pixels to perform rewriting.
When the Charged Electrophoretic Device (charged particles) described above is used or when the device having spontaneous polarization is used, the movement of the charges or the polarization inversion is caused in the device by rewriting, so the amount of charges stored in the capacitor changes.
When the amount of charges reduces before a sufficient change is caused in the device, a write state does not reach a desirable state and thus remains at an intermediate state.
Such a phenomenon appears more significantly as the resolution of a display increases. This is because an area for forming the auxiliary capacitor becomes smaller.
When the phenomenon is viewed in an actual device structure, the problem becomes clearer.
FIG. 3 is a cross sectional view illustrating a pixel electrode in a case where a thin film transistor (TFT) matrix array is formed on a glass substrate.
A gate electrode 11 and an auxiliary capacitor (Cs) electrode 12 are formed on a glass substrate 10, and then a gate insulating film 13 is formed thereon. After that, an amorphous silicon layer 14, an ohmic contact layer 18, a source electrode 15, and a drain electrode 16 are stacked in the stated order. A channel protective film 17 is formed on an uppermost surface.
An auxiliary capacitor Cs includes the auxiliary capacitor electrode 12, the drain electrode 16, and the gate insulating film 13 sandwiched therebetween.
In a case of a high-resolution display, that is, a display whose pixel area is small, an area of the auxiliary capacitor electrode 12 is small, so the auxiliary capacitor Cs is small. Therefore, the amount of charges stored with respect to the same applied voltage reduces.
When the auxiliary capacitor proportionally becomes smaller with a reduction in area of a display device included in a pixel, there is no problem. However, an area occupied by a TFT switching device and a matrix wiring which are formed on the same substrate does not reduce proportional to the auxiliary capacitor. Therefore, the auxiliary capacitor is reduced to a size equal to or smaller than a pixel area ratio as the resolution of the display increases.
In order to be able to normally drive a pixel even when the resolution increases, the followings are expected.
(1) A potential of the auxiliary capacitor electrode 12 is modulated to increase the amount of charges for retention.
(2) A process design rule is changed.
(3) A driving voltage is increased.
However, in the case of (1), a cost of a peripheral circuit increases and a driving method is limited. In the case of (2), it is necessary to improve alignment precision among wirings or layers. In the case of (3), an increase in cost of the peripheral circuit is caused by the improvement of a withstanding voltage. It is difficult to apply any method to a high-resolution display.